Cerrosafe

Precision measurement of the diameter of holes is one of the trickier problems for neophyte machinists. Calipers are worthless for the job, split-ball gages require developing a "feel", and inside micrometers won't go below 0.2". Plug gages provide accuracy and can measure below the rim of the hole but are expensive, especially so given their infrequent use.

However, there is a relatively cheap way to get the job done. Gunsmiths are often faced with the job of measuring the internal diameters of the chamber in a rifle. They get the job done by making a cast of the chamber, extracting the cast after the metal has cooled and measuring the cast rather than trying to work inside the chamber. The material they use is something called Cerrosafe.

Cerrosafe is an alloy with a low melting point. It is a non-eutectic mixture consisting of 42.5% bismuth, 37.7% lead, 11.3% tin, and 8.5% cadmium that melts between 158 °F (70 °C) and 190 °F (88 °C). It is useful for making reference castings due to its well-known thermal expansion properties during cooling. The alloy contracts during the first 30 minutes, allowing easy removal from a mold, then expands during the next 30 minutes to return to the exact original size. It then continues expanding at a known rate for 200 hours, allowing conversion of measurements of the casting back to those of the mold.

Cerrosafe is heavy, .341 lbs./in^3 (9.4 g/cc) and costs around $30/lb from gunsmith suppliers or Amazon.

The picture shows a half pound slug. In the background is the stainless rice measuring cup in which I melt it.

Given the nature of some of the component metals in the alloy, it's not a good idea to overheat it when melting it. Use a double boiler or a heat gun rather than a torch. A temperature controlled hot plate is also a possibility.

Beyond its use for dimensional casts, Cerrosafe can sometimes be used for securing small, difficult-to-clamp workpieces for light machining operations. Many options are possible but, as an example, carve a suitably-sized depression in a bit of scrap, insert workpiece and fill remaining space with Cerrosafe.

On edit: I neglected to mention that Cerrosafe is completely reusable. When your last cast is no longer needed, simply melt it down and tuck away for the next time it's required.

Our inspection dept used a similar product for checking bevel gear moulds etc and it worked very well due to its low shrinkage property's.
After a worker was dismissed for stealing it to make of all things chess pieces they switched to a resin based system that was also effective.

It's a useful tool. One of the things that pin gauges, ball gauges, T gages and other instrument will not tell you is how not-round the hole is. For example, a pin gauge placed in a square hole will tell you the same thing as one placed in a round hole It fits nicely in both places, but leaves a lot out of the measurement of the square (or other) shaped hole. The cast material can tell you a lot more than you may want to know about the hole you're interested in. On the downside, if the hole is belled, you may not be able to get the cast piece out at all w/o remelting it.

I've also used it for filling thin tubing so it can be clamped in a chuck,as well as for making tight bends in small tubing, as a removable core for casting two-part plastics, I've also been told of people plating copper or other metals to the surface, and melting it away, leaving a thin formed part for use.

I has one of the shop leakers who always went through coffee like it was free, so one day, I made a spoon out of Wood's alloy! The look in his face was priceless, seeing half a spoon in his hand! It got better when I took it from him, took a sip, and said, "Damn. Needs more spoon." and tossed in the piece he had in his hand!

I've also used it for filling thin tubing so it can be clamped in a chuck,as well as for making tight bends in small tubing, as a removable core for casting two-part plastics, I've also been told of people plating copper or other metals to the surface, and melting it away, leaving a thin formed part for use.

Yes, one of my main reasons for buying some was to make bends in very small copper tubing I was using for steam piping on a model engine. It worked very well for that purpose.

I hadn't thought of the plating method for making thin shells but I'll certainly keep that in mind for future projects; sounds very ingenious.

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